The present invention relates to visual inspection of reticles.
Electronic wafers are usually manufactured using a layer-by-layer methodology, with each layer""s manufacture including various etching and deposition processes, which are applied with the aid of a reticle and/or a mask. To create a complete layout on a wafer, the reticle is stepped sequentially over the wafer and at each step, the pattern of the reticle is transferred to the wafer.
The reticle itself is the end product of a complicated design process in which the entire layout of the wafer is determined and tolerances are set for the manufacture process. Nevertheless, it is often the case that the final manufactured wafer deviates from the design of the reticle. The reasons for the wafer being different from the design of the reticle may include defects in the reticle manufacture and unexpected interactions between the reticle design and the process used for the wafer manufacture. Therefore, reticles are typically inspected both by the mask shop manufacturing the reticle and at the semiconductor fabrication plant (Fab). According to current practice, reticles having non-repairable defects thereon are not shipped by the mask shop or are returned by the Fab, and never used. Due to the rapid reduction in design rules (the smallest resolvable feature size), manufacturing reticles is expected to become more difficult and the price of reticles is expected to rise. Therefore, it will become more desirable to use a reticle even if it has defects in its manufacture, providing however, that such defects do not invalidate the manufactured wafer.
One method of determining which defects cause problems in wafer manufacture is to simulate the physical processes which are involved in wafer manufacture. When a reticle is inspected after its manufacture, any detected defects are analyzed based on these simulations to determine if they will have an adverse effect on the wafer manufacture. If the adverse effect is within certain tolerances, the reticle is accepted. These simulations may also be used to generate a set of rules that govern which defects in a reticle are acceptable and which are not.
A critical problem with this approach is that the physical process must be understood in order to be simulated. Specifically, a simulation model of the stepper and fabrication process are first constructed. These models require expertise and knowledge of each stepper-fabrication combination, including, for example, the type of stepper used, the technology used to print the wafer and the development process. A new model may be required for each new combination of stepper, exposure and other processing steps and/or parameters. In some cases, a physical understanding of the parameters of the process are not available. As a result, Fab technicians are often uncomfortable relying on a model instead of on the actual equipment used in the Fab.
One object of some embodiments of the invention is to provide a method of reticle inspection which takes into account physical processes involved in printing a wafer based on the reticle. Preferably however, a detailed knowledge or analysis of the physical processes is not required.
One aspect of some preferred embodiments of the present invention relates to the generation of a test reticle comprising a plurality of pattern-features, generating a wafer using the reticle and determining which defects in the reticle result in unacceptable defects in the wafer and which do not. Throughout this specification, the term xe2x80x9cpattern-featuresxe2x80x9d is a short hand to signify either or both of xe2x80x9cdesign featuresxe2x80x9d and xe2x80x9ctest defects.xe2x80x9d Design features relate to features that correlate to the design of the circuitry to be printed. Test defects relates to defects which are intentionally introduced onto the test reticle. Both design features and test defects can be inspected for transferability. For example, design features can be inspected for the quality of their transfer, and test defects can be inspected to check whether they xe2x80x9cprintxe2x80x9d onto the wafer.
In a preferred embodiment of the invention, any defects which transfer from a reticle to the wafer are considered unacceptable. Defects in a reticle which do not transfer to a wafer may be considered acceptable. The determination may be absolute or it may include certain tolerances. Alternatively or additionally, even if the defect have an effect on the wafer layout, such defects are graded responsive to whether or not they affect the functionality of the wafer. Alternatively or additionally to using a test-reticle, the transfer of defects may be manually or automatically identified on existing reticles.
Another aspect of some preferred embodiments of the invention relates to training a neural network with the results of the determination of defect transfer. Preferably, the neural network is then used as part of a reticle inspection process, whereby detected defects on the reticle are analyzed using the neural network to determine if they transfer, as defects, to the manufactured wafer.
Another aspect of some preferred embodiments of the invention is that the entire process of the reticle testing can be done without any in-depth knowledge of the printing, development and/or fabrication processes involved.
A neural network training method, in accordance with a preferred embodiment of the invention comprises designing and manufacturing a test reticle, printing the circuit onto a wafer and checking the generated wafer for defects. The defects in the wafer are then matched to defects in the reticle using the known coordinates of the designed test defects. Additionally, the reticle can be inspected for additional unintended defects and their coordinates can be stored in the system""s memory. The wafer is then analyzed to see if the defects transferred as unacceptable defects onto the wafer. In some cases, unacceptability is simply that the defect transferred to the wafer, in other cases, unacceptability may be dependent on the functional effect of the transferred defect. In some cases, a defect for one use, may not be a defect for another, less demanding use. In a preferred embodiment of the invention, the neural network is trained by associating particular defects in the reticle with the acceptability of the resulting wafer.
Preferably, the test reticle used is a specially designed test reticle which contains a variety of predetermined pattern-features, some of which may be defective pattern-features. Additionally, the test reticle preferably includes especially designed defects. Alternatively, reticle-wafer sets that are known to contain defects may be used for training. Preferably, the defects are repeated for various dimensions and/or tolerances of the defect. Alternatively or additionally, the defects are repeated on the reticle so that a statistical evaluation of the defect transfer probability can be obtained. Alternatively or additionally, one or more wafers are generated using a plurality of different reticles and/or a plurality of different focus-exposure settings and/or a plurality of different process parameters, so that such statistics may be determined.
As noted above, in a preferred embodiment of the invention, the test reticle includes both a defected and a defect free example of each of a plurality of pattern features and a plurality of designed defects. Then, when analyzing the exposed wafer, the transfer of the defective patterns may be compared to the transfer of the non-defective patterns. Additionally, the wafer can be inspected according to the coordinates of the designed defects to determine whether they had transferred onto the wafer.
In a preferred embodiment of the invention, a combinatorial set of defect cells is produced, including, feature type, defect type and/or defect features. For example, a defect type xe2x80x9cnon constant widthxe2x80x9d may be matched with a feature xe2x80x9c1 micron conductorxe2x80x9d and a defect feature xe2x80x9cvariation greater than 10%xe2x80x9d. Each such cell preferably includes a defected feature-pattern and a defect-free feature pattern. In a preferred embodiment of the invention, a defect cell is not spatially contiguous. In a preferred embodiment of the invention, the defected feature patterns are arranged in order in one part of the reticle and the defect-less feature patterns are arranged in a corresponding order in a different part of the test reticle. When the exposed wafer is inspected, the two types of patterns may be optically scanned serially or in parallel and compared to each other.
In one preferred embodiment of the invention, an independent training procedure and/or test reticle is used for each manufacturing process.
In a preferred embodiment of the invention, defects in the wafer are detected by automated inspection. Preferably, the inspection utilizes an optical or electron microscope. For example, the wafer can be first inspected using Orbot WF 7xx(trademark) series wafer inspection system, available from Applied Materials(trademark) inc. of Santa Clara, Calif. Such an inspection will provide a defect map of suspected locations on the wafer. Preferably, the wafer also undergoes a review to verify that the suspected locations are indeed defective. Such a review can be done using, for example, the SEMVision(trademark) electron beam review station, also available from Applied Materials(trademark). The use of SEMVision(trademark) is highly recommended as it is a deterministic tool that can be used to confirm the output of the Orbot WF(trademark), which is of a statistical nature. Alternatively or additionally, destructive inspection methods, such as cross-section transmission electron microscope (TEM) and focused ion beam (FIB), can be employed.
In a preferred embodiment of the invention, when the neural network is trained, a particular defect is associated with various characteristics of the process for which the mask and/or wafer are used. In one example, the characteristics include the type of etching. In another example the characteristics include the clock speed of the device produced on the wafer.
In a preferred embodiment of the invention a table is maintained in which each defect and variations thereof are associated with a suitability for various uses of a wafer generated from a reticle with that defect.
In a preferred embodiment of the invention, a pattern matching computer utilizing pattern matching software and/or dedicated hardware may be used to perform pattern matching instead of a neural network.
A reticle inspection method in accordance with a preferred embodiment of the invention comprises, generating a reticle, inspecting the reticle for possible defects and analyzing the defects with a trained neural network to determine if the defects transfer and affect the manufactured items. Alternatively, the entire reticle may be analyzed, not only suspected defects.
In one embodiment of the invention, the quality of a process is periodically assessed by comparing the defects in resulting wafer with defects in a reticle. Preferably, this assessment may be used to update the training of the neural network.
There is therefore provided in accordance with a preferred embodiment of the invention, a method of reticle inspection, comprising:
generating a test reticle comprising a plurality of test pattern-features thereon;
exposing a wafer using the reticle; and
determining transferability of at least one of said plurality of pattern features from said reticle to said wafer.
Preferably, the method comprises training a reticle inspection system to associate a test pattern feature with a consequence of said transfer. Preferably, training comprises training a neural network. Alternatively or additionally, said test reticle does not define an operational layer of an operational integrated circuit. Alternatively or additionally, said pattern feature represents a printing on a reticle which is supposed to meet a design rule and wherein determining a transfer comprises determining if the pattern feature transferred in accordance with the design rule. Preferably, said pattern feature represents a defect in manufacturing a reticle in accordance with said design rule.
In a preferred embodiment of the invention, determining comprises determining a consequence of said transfer. Alternatively or additionally, said plurality of pattern-features comprise optical proximity correction (OPC) patterns. Preferably, said plurality of test pattern features comprises a plurality of pattern features corresponding to a single type of defect at a plurality of degrees of severity. Alternatively or additionally, said plurality of test pattern features comprises a plurality of pattern features corresponding to a plurality of different defect types. Alternatively or additionally, said plurality of test pattern features comprises a plurality of copies of a single pattern feature.
In a preferred embodiment of the invention, the method comprises:
inspecting a manufactured reticle to detect one or more pattern defects; and
comparing each of said detected pattern defects to said test pattern-features.
Preferably, the method comprises approving said manufactured reticle if said test pattern feature to which a detected defect is compared, does not transfer.
There is also provided in accordance with a preferred embodiment of the invention, a method of reticle inspection, comprising:
providing a reticle; and
matching one or more pattern-features of said reticle to a database of defect pattern-features, each defect pattern-feature associated with a consequence of said defect feature.
Preferably, the method comprises inspecting said reticle to determine one or more possible defects in said reticle, wherein said matching comprises one or more pattern-features which correspond to said possible defects. Preferably, said inspecting comprises comparing said reticle against a design of said reticle.
Alternatively or additionally, said consequence comprises a transfer of a defect onto a wafer manufactured using said reticle. Alternatively or additionally, comprises a non-transfer of said defect. Alternatively or additionally, said consequence comprises an effect on a functionality of a wafer manufactured using said reticle.
In a preferred embodiment of the invention, said matching comprises matching said one or more pattern-features to a database associated with a particular wafer manufacture process. Alternatively or additionally, said matching comprises matching said one or more pattern-features to a database associated with a particular mask manufacture process.
In a preferred embodiment of the invention, said matching comprises matching using a neural network, which database is embodied by said neural network.
There is also provided in accordance with a preferred embodiment of the invention, a production reticle inspection system comprising:
a training engine receiving data of known test defects produced on a test reticle and detected defects found on a test wafer, and providing training results correlating said detected defects to said test defects to identify test defects which have been transferred to the test wafer and test defects which have not been transferred to the test wafer;
an inspection result module receiving manufacturing defects data of defects detected on said production reticle by an inspection system;
a print/no print module receiving the manufacturing defects data and the training results, and providing a defect report identifying which of the manufacturing defects is likely to be transferred to a manufactured wafer.
Preferably, said print/no print module comprises a neural network. Alternatively or additionally, said print/no print module comprises a pattern matching computer.
In a preferred embodiment of the invention, the system includes a re-train feedback loop between a defect review module and said training engine.
There is also provided in accordance with a preferred embodiment of the invention, a test reticle comprising:
a reticle substrate; and
a plurality of feature-patterns formed thereon,
wherein said plurality of feature-patterns comprises defective feature patterns which may transfer to a manufactured wafer.
Preferably, said defects comprise defects in a reticle generation process.
In a preferred embodiment of the invention, the reticle includes a plurality of optical proximity correction (OPC) feature-patterns.
Alternatively or additionally, said reticle comprises a phase shift mask.
Alternatively or additionally, said defects comprise defects in a wafer manufacture process.
In a preferred embodiment of the invention, feature patterns of said plurality of feature patterns are selected responsive to a particular wafer manufacture process. Alternatively or additionally, feature patterns of said plurality of feature patterns are selected responsive to a particular mask manufacture process. Alternatively or additionally, feature patterns of said plurality of feature patterns comprise a plurality of copies of a single feature pattern. Alternatively or additionally, feature patterns of defect feature patterns comprise a plurality feature patterns, each corresponding to a different defect type. Alternatively or additionally, feature patterns of defect feature patterns comprise a plurality feature patterns, each corresponding to a different severity level for a single defect type.
There is also provided in accordance with a preferred embodiment of the invention, a reticle inspection system, comprising:
a database associating a plurality of feature patterns with a consequence of said feature pattern;
an image input; and
a matcher which matches said input image with said database; and
a consequence output which outputs a consequence associated with said image input.
Preferably, said database and said matcher are embodied as a neural network. Alternatively or additionally, said consequence comprises a transfer of said feature pattern as a defect, from a reticle to a wafer. Alternatively or additionally, said consequence is associated with a wafer manufacture process definition.
In a preferred embodiment of the invention, the system includes a trainer, which maintains said database. Alternatively or additionally, the trainer generates said database. Alternatively or additionally, the trainer comprises:
a defect image input;
a reference image input; and
a database interface which updates said database with a consequence, responsive to one or more differences between said defect image and said reference image.
Preferably, the system comprises a consequence input for entering a consequence. Alternatively or additionally, said consequence comprises an indication if said defect transfers to a wafer or not. Alternatively or additionally, said consequence is associated with one or more wafer manufacturing parameters.